It allows you to view the planet's current appearance and move forward and backward in time, in increments ranging from seconds to years. Sky & Telescope Magazine has a very good app for iOS users called JupiterMoons (developed by the SkySafari app team). (Image credit: SkySafari App)Īnother option is to choose an app that focuses exclusively on Jupiter. Use your astronomy app to find out when to look for it. Jupiter's 10-hour rotation period causes the spot to be visible for only a few hours at a time, roughly every second evening. And a good telescope will let you see the Great Red Spot.
A larger telescope will show the brown equatorial bands around the planet. In binoculars or a small telescope, Jupiter's four largest moons ― Io, Europa, Ganymede and Callisto― become visible to either side of the planet. The SkySafari app has an algorithm that corrects for this, but some of the other sky-charting apps I tested did not. It varies through the year, but right now, it's delayed by about 37 minutes. The light needs time to travel all the way to Earth. Jupiter is far enough away (more than 424 million miles, or 682 million kilometers) that we don't see events there in real time. If your app is set to the current time, it will show Jupiter as it appears in your telescope right now. However, the better apps such as SkySafari 5 present Jupiter as a complete globe that rotates at the correct rate. Many sky-charting apps show Jupiter as a photographic image with the red spot visible, which might fool you into thinking it's always there. A mobile astronomy app is a perfect way to find out when to see it. Thus, the spot is not visible every night. Jupiter rotates quite quickly - once on its axis every 10 hours - and the spot takes about 3 hours to traverse the planet's disk. The Great Red Spot's oval is large enough to hold two to three, and it is visible in backyard telescopes. A persistent spot on Jupiter was reported even earlier, by Giovanni Cassini, from 1665 to 1713 - but no one is sure whether that was the same storm we see today. The famous Great Red Spot (or GRS) on Jupiter is a cyclonic storm that has been raging on Jupiter for at least 185 years. (Image credit: SkySafari App) Spotting the Great Red Spot The SkySafari 5 app can display the path of a selected object. If you observe the planet every week or two, you'll see Jupiter moving away from Spica until June 10, then approaching it again until early September, after which it pulls away to the east. Jupiter's position near Spica this year offers an excellent chance to see how planets got their Greek name asteres planetai, or "wandering stars." From February through May, Jupiter's regular eastward journey through the distant background stars is reversed by the parallax effect of Earth's faster motion. After mid-September, due to Earth's orbital motion, Jupiter will disappear from view while it's near the sun during solar conjuction, and then become a morning object at year-end.
By August, the planet will be sinking into the western twilight after sunset and shining through twice as much atmosphere. But try to look now, while the planet is higher in the sky and shining through a thinner layer of the Earth's distorting atmosphere. Jupiter will be visible in evenings for the next few months. By the time Jupiter sets in the west before dawn, the rotation of the sky has moved Spica upward to the left of the planet. Despite Jupiter's great distance, its large globe reflects a lot of sunlight: it's second only to Venus in brightness among the planets, and it outshines every star in the night sky. It's easy to tell the planet from the star. Virgo's brightest star, Spica, is about 10 degrees (an outstretched fist's diameter) below Jupiter. In May 2017, Jupiter is sitting in the southeastern evening sky, within the constellation of Virgo.
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